Process and device for reading out binary information magnetically stored in a transparent magnetic layer

ABSTRACT

The invention is based on a process for reading out binary information magnetically stored in a transparent magnetic layer (1) in which the signals supplied by a magnetic read head (3), pressed against the magnetic layer, (1) is preamplified, frequency-filtered and evaluated. The preamplified signals are filtered in different filter frequency (10a-10d) ranges and differentiated (11-14), and the signal with the most plausible bit sequence is selected from these signals of the various frequency ranges for further processing.

CROSS-REFERENCE TO RELATED APPLICATION

This application is related to the commonly assigned U.S. patentapplication Ser. No. 08/913,695, filed Sep. 10, 1997 entitled "PROCESSAND DEVICE FOR READING MAGNETICALLY RECORDED SIGNALS" (now allowed) anda 371 of PCT/EP 96/01058, filed Mar. 13, 1996.

BACKGROUND OF THE INVENTION

The invention is a process and a device for reading out binaryinformation magnetically stored on a transparent magnetic layer.Transparent magnetic layers have recently become well known inphotographic films, where they are used to record information forillumination and/or processing of photographic films according to theinvention and to maintain data for access in devices used in processing.Because of the requirement that photographic films can not be alteredessentially in their capacity to store images and thus in theirtransparency, these magnetic layers can contain only very littlematerial. The magnetic fields of recorded signals are correspondinglyweak; they may be lower by a factor of 200 than fields for customarysound and data recordings on nontransparent magnetic tapes or disksdesigned exclusively for that purpose.

Special measures must thus be taken for capturing and processingmagnetic signals which make up the kind of magnetic code as, forexample, described in the U.S. Pat. No. 4,987,439. Special amplificationand means for evaluation have been developed as, for example, disclosedin the U.S. Pat. No. 4,964,139 for processing weak signals produced bymagnetic heads. There a circuit with a single filter 18 is describedwhose purpose is to eliminate interference signals like the intrinsicnoise of a pre-amplifier. It is extraordinarily difficult to tune afilter to multiple interference frequencies caused by various differentrecording speeds or densities.

SUMMARY OF THE INVENTION

The purpose of the invention is therefore to create an evaluation devicesuitable for the evaluation of extremely weak and/or varying magneticsignals.

This task is solved by

A process comprising the steps of filtering the preamplified signals atdifferent filter frequency ranges, and differentiating the filtersignals, such that those signals of the different filter frequencyranges which have the most plausible sequence of extreme values may beselected for further process.

Through development of the evaluation device according to the inventionit is no longer necessary to separate the signal read out by themagnetic head and amplified by the amplifier with a single frequencyfilter from interference signals. Now the signal read-out and amplifiedsignal are subjected to filtering through a number of various frequencyfilters and the preferred impulses are selected as bits on the basis ofplausibility considerations and agreement from the different filteredsignal sequences. Useful configurations of the invention bear on thekind of filtering sequence and differentiation, suppression ofinterference signals, as well as a device for carrying out the process.

Details concerning the embodiment of the invention are described indetail in Figures.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 a total schematic of the construction of the circuit for signalamplification and evaluation;

FIG. 2 the transmission characteristics of the frequency filters used;

FIG. 3 the configuration for time delay of filtered signals;

FIG. 4 the principle of differential subtraction of original and delayedsignals;

FIG. 5 examples of the route of differently filtered and differentiatedsignals as well as for testing established extreme values and theirdigitization into bits.

FIG. 6 Examples of the evaluation of bit values as bit lengths infiltered and differentiated signals according to FIG. 5.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

In FIG. 1, a photographic film strip with a very thin magnetic layer isdesignated as 1. It is led past a magnetic read head 3 with a coil 4 inthe direction of the arrow 2 in a film processing apparatus, for examplea photographic printing machine. Coil 4, connected on one side toground, is connected on its other end with the negative input 5a of anoperational amplifier 5. Its output 5c is connected with the input 5a bya feedback resistance 6, which has a very high resistance of some tenMΩ. Operational amplifier 5 has a very high no-load amplification A₀ ofmore than 10⁶ and an insignificant intrinsic noise. The positive input5b of amplifier 5 is connected via positive feedback to a resistancenetwork containing a resistance 6a so that extensive compensation ofinternal resistance of the magnetic head coil 4 is achieved. With such arelationship of feed-back resistance 6, no-load amplification A₀ of morethan 10⁶, and a magnetic head coil 4 with the lowest possibleresistance, the magnetic head can in practice be used in short-circuitoperation.

Output 5c of pre-amplifier 5 is connected with an analog-digitalconverter 7 which digitizes the amplified analog magnetic head signalwith a sufficiently high sampling frequency very strongly dependent onthe frequency response and relative speed between the film 1 and themagnetic head 3. For example, a digitization frequency of at least 250kHz is required with a film transport speed of 30 cm/sec at the magnetichead. This corresponds to a cycle time of 4 μs. With a lower transportspeed of the film the sampling frequency of the analog-digital convertercan correspondingly be reduced. The digital output signal in circuit 7ais directed to a data buffer 8 for asynchronous processing. Its outputis connected via circuit 9 with 4 processing channels arranged inparallel at the beginning of which are frequency filters 10a, 10b, 10c,and 10d. Their transmission characteristics are illustrated in FIG. 2.It can be seen from this that these frequency filters are low passfilters which are substantially impassable to the frequency spectrumlying above the cut-off frequency. Lower frequencies within a certaintransition area are transmitted in almost full strength near the cut-offfrequency.

For each of the four diagrams arranged above each other in FIG. 2, whosex-axes are shown in logarithmic scale, a transmission curve of thefilters 10a-10d is represented which rises from an initial frequency tothe maximum transmission and at each cut-off frequency quickly falls toalmost 0. The cut-off frequencies of filters 10a to 10d differ by aconstant factor, specifically by the whole number 2. That means that thecut-off frequency of filter 10a in the example given is at 9 kHz, forfilter 10b at 18 kHz, for filter 10c at 36 kHz, and for 10d at 72 kHz.These cut-off frequencies must clearly lie below the sampling frequencyof analog-digital converter 7; in particular the highest filter cut-offfrequency of about 72 kHz is lower by a factor of 3 than the samplingfrequency of 250 kHz.

Naturally a greater number of filter channels could be implemented, butthe described exemplary embodiment with cut-off frequencies between 9and 72 kHz may yield a clear difference in the frequency content of theoutput signal. In comparison with the various filtered signals, there isindeed a predominance of the proportion of frequencies which werefiltered out in the next lower signal.

According to FIG. 1 the signals amplified and filtered for frequency aredirected farther to differentiation devices 11 to 14. Normallydifferentiation in signal processing is accomplished by insertion of acapacitor. This kind of differentiation has considerable disadvantages,however, in a signal with strong noise. Differentiation by means of acapacitor yields amplification of high frequency noise, which in thiscase is not desirable. Production of a gradient signal from theamplified and filtered signal is therefore accomplished in the mannerdescribed in FIGS. 3 and 4. In accordance with FIG. 3 a delay member 19is inserted in the path of the output signal U_(e), which produces asignal U_(a) delayed by an amount τ. The amount of time delay τ must bedetermined with consideration of the amount of time required for therise of the signal from a minimum to a maximum, so that the greatestpossible amount of change in the signal can be evaluated in the shortestpossible time by differential formation in differential calculationdevice 19a. Experimental testing has shown that these premises arefulfilled best when time delay τ is chosen dependent on the wave lengthof the associated filter cut-off frequency, in particular approximatelydouble the period of oscillation of the associated cut-off frequency.

In FIG. 4, a rise of the filtered signal U_(e) is illustrated in 4a; inFIG. 4b the signal U_(a) is time-delayed by an amount τ. In FIG. 4cthese two signals U_(e) and U_(a) are illustrated, and FIG. 4d shows thedifference U_(e) -U_(a). This differential signal indicates themagnitude of the change of signal in the range of τ, therefore adifferential signal of low noise which represents the average climbgradients of the filtered signal.

In FIG. 5, signals originally recorded at the output of theanalog-digital converter 7a are compared with filtered anddifferentiated signals of different channels at the output of thedifferential calculation devices 11a, 12a, 13a, and 14a. It is worthnoting that the recording speed rises across the whole length of thediagram or the sampling speed declines and with it the interval betweenpositive maximums increases both in curve 7a and in curves 14a-11a. Thiseffect can, for example, be brought about by the starting of the filmtransport mechanism and the starting of the signal recording device in abattery-operated camera. The lower relative speed also led to anamplified noise in curve 14a. The noise component is especially highhere because the time-delayed noise component of the signal clearlysinks under the cut-off frequency of 72 kHz. This noise component can,however, be eliminated for the most part by the lower cut-offfrequencies of filters in channels 11-13; particularly in curve 12a itis obviously no longer interfering. On the other hand extreme values aresuppressed under the zero line by the still lower cut-off frequency offilter 11, so that this frequency is eliminated from the laterevaluation.

Before the variously filtered and differentiated signals in channels 11and 14 are compared and the most plausible signals are evaluated as bitpatterns, a process of elimination of signals which on the basis oftheir position or size do not belong in a bit pattern is carried out ina further stage 15a, 15b, 15c, and 15d. First it is established in eachof the circuits 15a-15d which periods of oscillation an acceptable bitlength can have. This naturally depends on the dominating frequency ofthe signal after the process of filtering and differentiation.Experimental tests have established that an extreme value is acceptableor plausible as part of a bit pattern when the recognized bit lengthcorresponding to a basic wave length, namely the interval of twoidentical positive extreme values with an intermediary extreme value ofreversed sign, lies between the single and the doubled period ofoscillation. If, for example, the cut-off frequency of filter 10damounts to 72 kHz, which corresponds to a period of oscillation of about14 μS, it will be accepted as a bit length if the interval of twopositive extreme values lies between 14 and 30 μs. As additionalcriteria for the elimination of not unambiguously identifiable bitpatterns, the fact can be used that the time interval between twodifferently directed extreme values in relationship to the probable bitinterval is too small or that the amplitudes of two successivedifferently directed extremes is too small in comparison with theprevious extreme value differences.

The peak values found on the basis of tests with a subsequently addedpeak value detector 16a-d or signals recognized as extreme values arerepresented in FIG. 5 by vertical lines 20. This is accomplishedelectronically by digitizers 17a-d. This drawing also takes into accountthat the successive extreme values can be recognized only as part of abit pattern when they have alternating signs.

In FIG. 6, evaluations of the differentiated curves 11a-14a as in FIG. 5are illustrated with horizontal bars between two equidirectional,positive extreme values which correspond to the plausibilityrequirements of the interference signal corrector 15.

By comparison of the different filter signal curves 11a, 12a, 13a, 14a,etc. of the bars given there for the probable bit length 21 it can bedecided which of the bars 21 in this time range or signal range can berecognized as having the most secure bit patterns and then beillustrated as characterized by the black bar 22. Criteria already usedin stage 15 can in principle be used for this selection, whereby it mustbe decided which of the possible bit lengths from the different channelsbest fulfill the criterion. Other criteria can be used in a mannerwhereby either individually determined cut-off values are compared foreach channel or bit lengths occurring approximately at the same time arecompared. The following criteria have emerged as appropriate eitheralone or combined:

bit lengths 21 agree in different channels in timing and length;

the bit lengths lie between whole and doubled wave lengths of theassociated filter cut-off frequency; and

the interrogation of possible bit lengths proceeds from the higherfilter cut-off frequency to the lower until a bit length is found in theacceptable range.

In the bit patterns recognized as acceptable a decision is then made asto whether the bit represents a 0 or a 1 on the basis of the position ofthe opposite-directed bit lying between the two equidirectional extremevalues.

In place of the low-pass filters 10a-10d, narrow-band filters can alsobe used which cut off the bottom frequency range. Stronger emphasis onhigh-frequency signal components is also supported by the kind ofdifference formation in FIGS. 3 and 4.

The functions of components 10-18 in FIG. 1 can also be carried out by asingle appropriately programmed microprocessor or a digital signalprocessor.

Filtering the pre-amplified signals with different filter frequencyranges can also be carried out sequentially if appropriate memory forthe pre-amplified signal is provided from which a signal can beextracted for filtering.

The described pre-amplification of magnetic read-out head signals bymeans of short-circuit operation makes possible especially advantageousdifferential formation after frequency filtering.

There has thus been shown and described a novel process and device forreading out binary information magnetically stored in a transparentmagnetic layer which fulfills all the objects and advantages soughttherefor. Many changes, modifications, variations and other uses andapplications of the subject invention will, however, become apparent tothose skilled in the art after considering this specification and theaccompanying drawings which disclose the preferred embodiments thereof.All such changes, modifications, variations and other uses andapplications which do not depart from the spirit and scope of theinvention are deemed to be covered by the invention, which is to belimited only by the claims which follow.

What is claimed is:
 1. In a process for reading out binary informationmagnetically stored in a transparent magnetic layer, wherein signalsproduced by a magnetic read head, applied against the magnetic layer,are pre-amplified, frequency-filtered, and then evaluated, theimprovement comprising the steps of filtering the pre-amplified signalsat different filter frequency ranges, thereby to produce a plurality offiltered signal curves which contain images of the binaryinformation;differentiating the filtered signals; determining theposition and direction of extreme values in the individual filteredsignal curves; and selecting two successive extreme values of the samedirection in one of the filtered signal curves to obtain one of the bitsof the binary information; whereby the two extreme values are selectedin dependence upon their positions in, and the filter frequency rangeof, their respective filtered signal curve.
 2. A process according toclaim 1, wherein the pre-amplified signals before filtering aredigitized with a scanning frequency which is a multiple of the highestfilter frequency.
 3. A process according to claim 2, wherein thescanning frequency is at least three times the highest filter frequency.4. A process according to claim 1, wherein low-pass filters of differentcut-off frequencies are used in the filtering step.
 5. A processaccording to claim 4, wherein adjacent ones of said filter cut-offfrequencies differ by a constant.
 6. A process according to claim 5,wherein said filter cut-off frequencies differ by an integer factor. 7.A process according to claim 6, wherein the integer factor is a factorof
 2. 8. A process according to claim 1, wherein the filtering step iscarried out in a per-unit-time parallel manner with frequency filterseach having a different cut-off frequency.
 9. A process according toclaim 1, wherein the differentiation step is carried out by a signaldelay of the filter output signals by a time τ and by formation of adifferential between the original filter output signal and the delayedsignal.
 10. A process according to claim 9, wherein the time τ of thesignal delay is determined differently in each filter channel.
 11. Aprocess according to claim 10, wherein the signal delay time decreaseswith an increasing filter cut-off frequency.
 12. A process according toclaim 10, wherein the time τ in the respective filter channel isapproximately twice as great as the period of oscillation of theassociated filter cut-off frequency.
 13. A process according to claim 1,wherein the differentiated signals of the various filter channels areexamined as to the distance between consecutive, differently directedextreme values, and wherein any intermediary extreme values aresuppressed because of at least one of (a) too short a time periodbetween acknowledged extreme values in relation to the presumptive bitlength of a given channel, and (b) an amplitude that is too low incomparison with other extreme value differentials.
 14. A processaccording to claim 1, further comprising the step of determining thedistance between two adjacent extreme values of the same direction, andwherein the evaluation of the extreme values is dependent upon thisdistance.
 15. In a device for reading out binary informationmagnetically stored in a transparent magnetic layer, wherein there iscarried out a frequency filtering in a circuit for detection of binaryinformation in the signals from a magnetic read head which is applied tothe transparent magnetic layer of a photographic film and advanced inrelation thereto, the improvement wherein said magnetic read head iscoupled to a pre-amplifier, and wherein the pre-amplifier is coupled toa plurality of evaluation channels having various frequency filters, towhich a downstream connection is provided in the following order: adifferentiating device, a peak value detector, and a common selector.16. A device according to claim 15, wherein there are provided fourparallel evaluation channels, the filters of which are designed as lowpass filters having a constant factor of different cut-off frequencies.17. A device according to claim 16, wherein the constant factor of thelow pass filter cut-off frequencies is an integer.
 18. A deviceaccording to claim 17, wherein the integer is
 2. 19. A device accordingto claim 15, further comprising a digital computing means for thefiltering of the digitized signals, for the differential formation, forthe separation of interference signals, and for the peak valuedetection.
 20. A device according to claim 19, wherein the digitalcomputing means is a custom programmed microprocessor or digital signalprocessor, which compares the bits from the various channels and selectsone of these bit lengths to be correct in accordance with certaincriteria.
 21. A device according to claim 20, where in the bit lengthcorrespondence as to position is retrieved as the selective criterion.22. A device according to claim 20, wherein the bit length in differentchannels is retrieved as the selective criterion.
 23. A device accordingto claim 20, wherein correspondence as to bit length in a channelrelative to the length of the bit which was last assessed as valid isretrieved as selective criterion.
 24. The device according to claim 15,wherein the magnetic read head is operated through the pre-amplifier inshort circuit.